As water issues are becoming serious all over the world, water business is regarded as a huge market, and international competition in water business is accelerating. In Middle Eastern countries where there is no water source of surface water such as rivers and groundwater, or in regions in our country where water shortage is highly possible, seawater desalination techniques are introduced in order to secure water source, and large seawater desalination plants are installed.
Seawater desalination methods in actual use include a method called evaporation method for obtaining fresh water by evaporating seawater and a membrane method for filtering seawater at a high pressure using a reverse osmosis membrane (hereinafter referred to as RO membrane). In the membrane method, seawater whose pressure is boosted by a high-pressure RO pump is passed through an RO membrane, so that the seawater is desalinated. The desalination cost of the membrane method is less expensive than that of evaporation methods in general. Therefore, the membrane method is now widely spread in place of the evaporation method.
By the way, one-stage RO membrane separation processing may not reduce boron contained in seawater (4 to 6 mg/L) to a level satisfying a reference value (drinking water reference level 1.0 mg/L, or 0.3 mg/L defined by WHO). In such a case, there are some plants, in which alkaline agent is added to the water processed by the one-stage RO membrane, so that the water is processed in two stages of RO membranes (hereinafter referred to as two-stage RO membranes).
It should be noted that since boron exists as a weak electrolyte boric acid and changes according to pH, boron stopping rate of the second stage RO membranes is required to achieve a boron concentration corresponding to drinking water reference level of 1.0 ppm (0.3 ppm defined by WHO) by adjusting pH to 9.0 to 10.0.
In order to prevent calcium scale film with the first stage RO membrane, pH at the entrance of the first stage RO membrane needs to be adjusted to 6.5 to 7.0 by injecting acid such as sulfuric acid and hydrochloric acid (pH of seawater is about 8.0 to 8.5).
As shown in the expression below, the equilibrium position may be shifted to the left by adding H+ as acid, so that calcium carbonate can be dissolved.Ca2++HCO3H++CaCO3 